1. Field of the Invention
This invention relates to atomic filters in general and to passive atomic filters in particular.
2. Description of the Prior Art
Atomic resonance filters are optical bandpass filters that employ sharp atomic transitions to perform ultranarrowband (.about.0.001 nm) filtering of light. Further, the bandwidth is independent of aperture size and field of view. The properties of atomic filters make them ideally suited for the detection of weak laser signals against continuum backgrounds.
Atomic filters can be divided into two classes: active and passive. Promising active filters include sodium, thallium, rubidium, magnesium, and calcium. Active devices require power sources, usually in the form of optical radiation, to sustain filter operation. These sources add complexity and cost, and may reduce reliability. In several instances, the absorbed optical power has been shown to contribute filter noise.
On contrast to the large number of active filters, only two passive filter schemes have been developed. These filters employ rubidium and cesium atoms and they operate near 420 and 456 nm, respectively. Passive filters are advantageous for applications that stress low level light detection, high reliability, low cost, and design simplicity. There are several important shortcomings with these passive atomic filters. First, these filters emit at wavelengths greater than 700 nm. Few photomultiplier tubes are available to receive these emissions. Second, there is no inherent sunlight rejection. Third, the optical bandwidths cannot be pressure broadened to accommodate Doppler shifted signals.
Therefore, a principal object of the present invention is to provide a passive filter which maximizes solar background rejection. It is also an object to provide a filter that emits int he red spectral region (below 700 nm) and has a variable optical bandwidth.